The most commonly used DRAM cell structure is the one transistor/one capacitor cell. This DRAM cell structure typically requires the deposition of three layers of conductive polysilicon: one layer for the gate of the transistor, one layer for the bottom storage node of the capacitor, and a third layer for the top storage node of the capacitor. The relatively complex process required to form modern DRAM cells causes practical incompatibility with standard logic processes that typically use only a single polysilicon layer.
Nevertheless, with the trend towards "system-on-a-chip" devices where memory and logic are placed onto a single chip, it is important to develop a DRAM cell structure that will be compatible with logic. There have been prior art attempts to design a DRAM cell structure that can store information without the benefit of a capacitor. An example of such a DRAM cell is disclosed in "A Novel Merged Gain Cell for Logic Compatible High Density DRAMs," by Mukai et al., Symposium on VLSI Technology Digest of Technical Papers, 1997, at 155. The DRAM cell disclosed in the Mukai et al. reference shows a single transistor structure that uses n+ and p+ regions formed in p-well and n-wells, respectively. Although this proposed DRAM cell design does address some of the problems of embedded DRAM design, the DRAM cell design proposed by the Mukai et al. reference requires very precise manufacturing processes to ensure that the DRAM cell will operate correctly. In addition, the fabrication process is still relatively complicated.
What is needed is a new design for a DRAM cell that can be used in embedded logic applications.